A plastic lens is lightweight and hardly broken as compared to an inorganic lens, and can be tinted. Therefore, the plastic lens has quickly come into wide use as an optical element such as spectacle lenses or camera lenses, and various resins for spectacle lenses have hitherto been developed and used. Among these resins, representative examples include an allyl resin obtained from diethylene glycol bisallyl carbonate or diallyl isophthalate; a (meth)acrylic resin obtained from (meth)acrylate; and a thiourethane resin obtained from isocyanate and thiol.
Recently, a urethane resin formed of isocyanate and alcohol which has a lower refractive index than the thiourethane resin has been developed (Patent Documents 1 to 5). This urethane resin has a lower refractive index than that of the thiourethane resin but is expected as a cheap lens material. For example, Patent Document 4 describes a urethane resin obtained from 4,4′-methylene-bis(cyclohexylisocyanate), which is an isocyanate, and trimethylolpropane propoxylate and trimethylolpropane which are alcohols.
In addition, Comparative Example 5 of Patent Document 6 describes an example of producing a urethane resin in which tolylene diisocyanate which is easily available at a low cost is used as isocyanate. Examples of commonly-used isocyanate include m-xylylene diisocyanate, 2,5-bis(isocyanatomethyl)-bicyclo-[2.2.1]-heptane, 2,6-bis(isocyanatomethyl)-bicyclo-[2.2.1]-heptane, 1,3-bis(isocyanatomethyl)cyclohexane, hexamethylene diisocyanate, isophorone diisocyanate, and 4,4′-methylene-bis(cyclohexyl isocyanate). The market price of tolylene diisocyanate is the cheapest among these commonly-used isocyanates. By using tolylene diisocyanate, a cheaper spectacle lens can be provided to the market, which significantly contributes to the industry. However, Comparative Example 5 of producing a urethane resin using tolylene diisocyanate describes that a cured product was not able to be obtained due to imperfect molding.